The present invention relates to a method for manufacturing a board tray, which method comprises moulding a blank of plastic-coated board into a tray comprising a bottom, side walls, as well as a rim flange, whereby, in the process of moulding, the blank is compressed in a direction transverse to the tray's side wall and rim flange such that some of the board's plastic coating is melted and flows into the compressions for stabilizing the tray so as to retain its shape. The invention also relates to a blank for applying the method, as well as to a tray obtained by the method.
Tray type containers of plastic-coated board material are used especially as consumer containers for food. A tray can be internally and/or externally plastic coated for protecting the tray against moisture coming from outside and/or from a packaged product, in addition to which the plastic coating may function as a heat sealing layer in the process of closing the tray with a cover sealed to its rim flange. In order to provide a sufficient barrier to water vapour, oxygen and/or aromas, the plastic coating can be multi-layered with an inner layer constituting the barrier and an outer layer serving as the heat sealing layer.
GB application publication 2123786 discloses an extrusion-moulded tray for foodstuffs, which is made of plastic-coated board material and which comprises a bottom, side walls and a rim flange, and which is closed with a cover sealed to the rim flange in a leak-proof manner. During the process of moulding, the corners of a rectangular tray have folded into pleats extending from the tray's bottom corners across the side wall and rim flange to the outer edge of the flange, as illustrated in FIG. 1 of the cited publication.
U.S. Pat. No. 4,026,458 discloses a similar container shaped by deep drawing from plastic-coated paperboard, in the corners of which the side walls and rim flange have become pleated. The shaping process involves the use of heat, which according to the cited publication sets the resulting container to its form, presumably in response to the softening and melting of a plastic coating during the shaping process.
EP publication 1115572 B1 discloses another description of a cardboard tray for food, which comprises similar corner pleats and which is closed impermeably with a heat-sealed cover. It has been said that the gas and/or diffusion proofness of the tray is provided by the cardboard's plastic coating layer having a thickness sufficient for levelling out some of the corner irregularities of the tray. However, the cited publication does not report required layer thicknesses, nor does the publication provide working examples to verify that an improved proofness has indeed been achieved. WO publication 98/00341 and the corresponding U.S. Pat. No. 6,093,460 also disclose a container, wherein corner pleats are levelled by means of a coating plastic for improving the heat-sealing properties of a cover. According to the cited publication, the viable coating can be obtained by using a polyester layer with a thickness of 40-45 μm.